Photoresist stripper composition and method of use

ABSTRACT

An improved process for removing crosslinked photoresist polymer from printed circuit boards which comprises contacting the printed circuit board with methylene chloride containing from about 5 to about 10 volume percent of a mixture of methanol and methyl formate. Each additive must be present at a minimum concentration of one volume percent. Stabilizers for the methylene chloride, such as epoxides, may be present at amounts no greater than about 0.5 volume percent.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 465,975, filed Feb. 14,1983, now U.S. Pat. No. 4,438,192.

BACKGROUND OF THE INVENTION

Printed circuit boards are manufactured by plating a thin layer ofcopper on an epoxy-glass cloth laminate board of uniform thickness. Apredetermined pattern of holes is then drilled to accommodate variouselectrical components. A film of partially polymerized photoresistplastic is deposited on the laminate over the copper plating.Photoresist films are commonly composed of acrylic resins, polyvinylcinnamates, diazo compounds, phenol-formaldehydes, or other similarfilm-forming materials. This film is further polymerized, orcrosslinked, by the action of ultraviolet light, into a hard chemicallyresistant film.

By masking with an appropriate glass or plastic material the resist filmis selectively hardened by the exposure to light in specificpredetermined areas, while the resist film in the masked areas is leftunchanged. The unchanged resist film is then dissolved in a "developer"by a solvent such as 1,1,1-trichloroethane or a solution ofbutoxyethanol and sodium carbonate or similar solutions. The copper inthe cleaned areas may then be removed by etching or additional copperand other metals may be plated thereon. In either event, it then becomesnecessary to remove the exposed hardened resist film from the laminate.

It is known that such resist films can be removed by the action ofstrong organic solvents, such as methylene chloride ortrichloroethylene.

It is known also that many paint and varnish removers are based onmethylene chloride. Many of these also contain an alcohol and one ormore other additives. Thus, U.S. Pat. No. 3,650,969discloses acomposition for removing paint which comprises dichloroalkanes of 1 of 2carbon atoms, an aliphatic monohydric alcohol containing 1 to 3 carbonatoms, and hydrogen fluoride and water. U.S. Pat. No. 3,600,322discloses a paint remover composition containing methylene chloride,methanol and a quaternary nitrogen cellulose ether.

Other patents employing methylene chloride with alkanols and variousother additives are U.S. Pat. Nos. 3,147,224; 3,075,923; 4,269,724 and4,278,557.

A method of removing resist from printed circuit boards is described inU.S. Pat. No. 3,789,007 wherein the board is treated with a mixture of85 to 97% by weight of methylene chloride with the balance beingmethanol.

Other methylene chloride-containing photoresist stripper compositionsare disclosed in U.S. Pat. Nos. 3,625,763; 3,813,309 and 3,625,763.

SUMMARY OF THE INVENTION

The present invention is an improved photoresist stripper compositionwhich contains methylene chloride, methanol and methyl formate. Thiscombination strips the resist at a faster rate than prior artcompositions. The preferred composition also provides a non-flashingmixture (blend) and gives better line definition than currentlyavailable commercial strippers.

DETAILED DESCRIPTION OF THE INVENTION

The improved methylene chloride formulation for use as a photoresiststripper contains methanol and methyl formate, each employed at aconcentration of from about one to about ten percent by volume based onthe total volume of the composition. The composition can optionallycontain a stabilizer for the methylene chloride such as a vicinalepoxide, e.g. propylene oxide. Amines are frequently used as an aid instripping, e.g. isopropylamine. Also useful are cyclohexylamine andtriethylamine. Such amines are known to be contained in some currentcommercial stripper formulations.

Blends according to the invention were tested in the following ways:

Test 1--Drop Test

A fifty microliter drop of the test blend (1-25% additive(s) inmethylene chloride) is placed on the crosslinked photoresist filmcontained on a printed circuit board. The solvent-film area is observedunder a 150× microscope to determine the time (in seconds) necessary tofracture and lift the film from the board. Lower times are preferred incommercial operations.

This test was run with a number of possible additive candidates usingcommercially-prepared photoresist-covered printed circuit boards. Theseboards utilized crosslinked photoresist film.* Results from these testsare shown in Tables I and II. All of the additives were tested in aninhibited grade of methylene chloride to which was added 0.1 volume %isopropylamine.

                  TABLE I*                                                        ______________________________________                                        Additive**                                                                              Drop Time (sec)                                                     (%)       5         7.5    10     15   25                                     ______________________________________                                        Me Form.  8.3       8.8    10.1   11.3 10.2                                   MeOH      10.3      10.0   9.6    10.4 11.5                                   i-PrOH    11.7      12.5   12.8   14.1 16.7                                   Me Acet.  13.1      13.2   13.5   15.9 20                                     ______________________________________                                    

                  TABLE II*                                                       ______________________________________                                        Additive**  Drop Time (sec)                                                   (%)         5      7.5        10   15                                         ______________________________________                                        Me Form.    11.8   12.5       12.8 14.5                                       Et Form     14.8   15.0       15.3 16.5                                       MeOH        12.4   12.3       12.2 11.4                                       i-PrOH      12.1   12.3       13.1 13.4                                       Me Acet.    14.2   14.9       15.0 18.0                                       THF         12.7   12.5       13.3 16.4                                       ______________________________________                                         *Table I shows results using a commercially available                         polymethylmethacrylate film and Table II a similar, but not identical         film, manufactured by the same company.                                       **MeOH = methyl alcohol                                                       iPrOH = isopropyl alcohol                                                     Me Form. = methyl formate                                                     Me Acet. = methyl acetate                                                     Et Form. = ethyl formate                                                      THF = tetrahydrofuran                                                    

Test 2--Spray Unit Test

A spray test was conducted utilizing a steel spray unit which was alaboratory scale version of the apparatus used in industry to stripresist film. Two-gallon quantities of test blends suggested from thepreliminary screening were used in the spray unit. Solvent spray at 20psig and 23°-24° C. was then directed onto the suspended boards for10-15 seconds. The boards were weighed before and after stripping.Weight differences in amount of removed photoresist were compared with astandard* using the formula: ##EQU1## A positive % value indicated morecomplete stripping and a negative value less complete stripping whencompared with the performance of the standard stripper blend.

The 4"×4" test boards were prepared by a commercial fabricator, using acommercially available photoresist resin** film and a test patternconsisting of numerous lines of varying widths and spacing. Theseboards, which were tin-lead plated, were ready for stripping asreceived.

Several compositions of methylene chloride containing different amountsof methanol and methyl formate were used in the above described spraytest. A timed spray of 10-second duration was used. Methanol and methylformate were each employed in amounts of 1.0, 2.5 and 4.0 volume percentin the methylene chloride (inhibited grade) based on the total volume ofsolvent and additives. Isopropylamine was present at 0.1 volume percentin each formulation total basis. Results of strippability of eachformulation are shown as percent better (+) or worse (-) than a standardmethylene chloride stripper in Table III.

                  TABLE III                                                       ______________________________________                                                            % MeOH                                                                1.0     2.5        4.0                                            ______________________________________                                        % Me Form. 1.0    -17.5     -10.7    +7.4                                                2.5    -1.6      +13.8    +13.2                                               4.0    +5.3      +4.4     +4.4                                     ______________________________________                                    

Evaporation and corrosion tests were also performed on thesecompositions.

Two tests were done for evaporation. In the first, 300 ml test solutionwas prepared, placed in a beaker at room temperature in the hood, andanalyzed at various times for additive composition using gaschromatography. Results are shown in Table IV. In the second test, twogallons of test solution were placed in the spray unit with the lidslightly opened. The unit was operated at 20 psig for a period of 60minutes. Again, the concentrations of the additives were evaluated atvarious times. For this test at the end of one hour the concentration ofmethyl formate was about 2.85% and that of the methanol was about 2.30%,the initial concentrations being 3.0 volume percent each, which showedonly slight loss of components from the mixture.

                  TABLE IV                                                        ______________________________________                                        *Composition (Vol.%)                                                          Time                           Volume                                         (min.)  MeOH         Me Form.  (ml)                                           ______________________________________                                        0       3.00         3.00      300                                             20     2.80         2.85      275                                             60     2.55         2.80      215                                             90     2.45         2.85      175                                            120     2.00         2.75      140                                            153     1.60         2.70      100                                            ______________________________________                                         *This composition also contained 0.1% isopropylamine by volume, but since     its levels were unimportant with respect to strippability its analysis wa     omitted.                                                                 

A copper corrosion study consisted of refluxing 100 ml of test solventblend with a 0.5×2.45 inch copper coupon for seven days. Triplicatedeterminations were run for each solvent blend. Gas chromatographyanalysis was done on each sample after the seven days. The corrosionrate of the copper coupon was expressed in mils penetration per year(MPY) using the following formula ##EQU2## Results for formulations of acommercial blend (A) and one according to the invention (B) under theconditions of the above test are shown below.

    ______________________________________                                        Formulation                                                                   (Vol. %)          Corrosion (MPY)                                             ______________________________________                                        A       MeOH        7.5   1.01                                                        IPA         0.1                                                       B       MeOH        2.5   0.1                                                         Me Form.    2.5                                                               IPA         0.1                                                       ______________________________________                                    

The amine can present a slight problem of copper tarnishing and solventdiscoloration if allowed to stand for a minimum of 12 hours at roomtemperature in the presence of the metal. This may not be a significantproblem with respect to the boards, however, since the solvent contactsthe copper circuit boards for only two to three minutes in actualcommercial use. Since industrial equipment typically uses copper piping,however, this phenomenon can affect equipment life. It is interesting tonote that use of a 0.1 volume percent triethylamine to replaceisopropylamine in the B formulation greatly decreased the solventdiscoloration problem in the room temperature copper corrosion tests.The amine adds nothing to the strippability characteristics of theformulation and if eliminated completely avoids the corrosion anddiscoloration of the copper.

The compositions according to the present invention are those containingfrom about 1 to about 10 volume percent each of methanol and methylformate, with the proviso that the total of methanol and methyl formatebe at least 5 and not more than about 10 volume percent of the totalcomposition. The composition may also contain an amine.

We claim:
 1. In a process for stripping crosslinked photoresist polymerwhich employs a mixture of methylene chloride and methanol theimprovement which comprises employing said mixture additionallycontaining methyl formate, and wherein the methanol and methyl formateare each present in an amount of from about 1 to about 10 volume percentof the total mixture and wherein the total amount of methanol and methylformate is at least about 5 and not more than 10 volume percent of thetotal mixture.
 2. The process of claim 1 wherein the methanol and methylformate are each present in an amount of from about 1 to about 5 volumepercent.
 3. The process of claim 2 wherein the methanol and methylformate are each present in an amount of from about 2.5 to about 5volume percent each.
 4. The process of claim 3 wherein the strippingmixture also contains an amine selected from the group consisting ofisopropylamine, cyclohexylamine and triethylamine as an aid instripping.